Method of etching a printing plate



June 9, 1964 c. s. MARTZ ETAL 3,136,671

METHOD OF ETCHING A PRINTING PLATE Filed May 2, 1960 3 Sheets-Sheet 1INVENTORS 2 CHARLES S. MARTZ MONROE R. HILLHOUSE ATTORNEYS June 9, 1964c. s. MARTZ ETAL METHOD OF ETCHING A PRINTING PLATE 3 Sheets-Sheet 2Filed May 2. 1960 :FIkME FIG.

ill ih. 47 4 EHI:

INVENTORS- CHARLES S. MARTZ MONROE R- HILL HOUSE BY FIG.

ATTORNEYS June 9, 1964 s. MARTZ ETAL 3,136,671

METHOD OF ETCHING A PRINTING PLATE Filed May 2. 1960 3 Sheets-Sheet 3 l29 FIG. 6

3| I r muflkflimu 24 INVENTORS 2o .1l. CHARLES s. MARTZ i n" MONROE R.HILLHOUSE ATTORNEYS United States Patent Filed May 2, 1960, Ser. No.25,921 Claims. 01. 156l4) This invention relates generally toimprovements in an etching machine and method of etching, and moreparticularly to improvements in an apparatus and method involving anetching bath in which a protective bath additive is utilized. V

The process of etching per se presents no problem. For example, amixture of water, nitric acid and air will etch zinc. One of the firstcommercial zinc etching methods used a rocking tray or tub in which awater-acid solution flowed back and forth across the plate, the platebeing exposed to air each time the solution passed over the plate. Inthis method, etching was relatively slow and because the bathcontainedno shoulder protecting chemicals, the lateral etch was almost as greatas the vertical etch. Consequently powdering between bites was necessaryin order to prevent the lateraletch.

Subsequently, paddle machines were introduced and plates were etched inevery possible position, from'vertical to horizontal. In such machines,powdering between bites was, of course, necessary. Air etching machineswere developed, but none were popular because they presented noadvantages over paddle systems.

Every photoengraver engaged in etching looked for ward to the day whendry powdering could be eliminated. A machine and process was visualizedwhich would allow powder to be put right in with the acid and water bathand yet provide side wall protection. When a bath additive wasdiscovered in the early 1950s that would protect side walls, a paddletype etching machine very similar to those in daily use at that time wasdeveloped and used. While these paddle machines were ideal forconventional powder etching, they were far short of perfect insofar asautomatic side wall protection was concerned.

In the above mentioned paddle type etching machines, the protectivechemicals in the bath are known in the trade as liquid powder or bathadditive. In powderless paddle or spray type etching machines in whichthe bath includes a bath additive, the bath is thrown onto the workplate. The zinc plate has an affinity for the oil constituting the bathadditive and it forms a film over the surface. This film is porous andit is removed by theaction of the acid through the pores'to'a minordegree. The major removal action however, is mechanical. The force ofthe paddle action beats the oil oil. Because the sides of the etchedimages are largely parallel to the force, the sides of the imagesreceive less paddle action and the film adheres to these sides 'while itis being beaten oif the open plate bottom areas.

The present machine and method constituting the invention has definiteadvantages over the prior known machines that splash the etching andprotecting solution over a suspended plate with paddle or spray IIOZizles. It is an important object of the present invention to eliminateall paddles in the etching machine, and to etch'the plate and protectthe side walls of the images while the plate is completely submerged inthe bath.

Another important objective is achieved by suspending the plate to beetched completely submerged in the etching bath at an angle to the planeof the bath surface and by rotating the plate while bombarding the plateface with a profusion of bubbles passed upwardly through Limited,Aurora, Mo., a corporation of.

' 3,136,671 Patented June 9, 1964 the bath so as to impinge and rollupwardly on the plate. The protective film formed by the bath additiveis in contact with the plate at all times because the plate is submergedin the bath. The air bubbles remove the bath additive in the open areasto be etched. In addition, the flow of the solutioh across the platecaused by the path ofthe bubbles and by the rotation of the plate alsocauses removal of the protective bath additive in the open areas to beetched, yetcompletely protects the side walls. i

With the above described superionmethod and machine, a much smootherside wall protection is provided than can be had by any other knownmethod or machine. These results are obtained because there is acontrolled flow over the plate produced by choosing the particular plateangle and rotation which provides greatest side wall uniformity. of theair bubbles in rolling upthe plate to ,be etched reduces the porosity ofthe protective bath additive on the side walls. I V 7 Another importantobject is to provide an etching machine in which there is completefreedom in bath replenishing because the precise depth of the bath is nolonger a critical factor as it is in paddle type or spray nozzle typemachines. Still another important objective is achieved by the placementof tubes in the bottom of the tank, such tubes having air holes formedand arranged so that bubbles emitting from the air holes will riseupwardly through the bath in a pattern relative to the rotative axis ofthe work plate in order to eliminate or minimize distortion in theprotection of side walls.

The present etching machine and method .has many advantages over thepaddle. or spray nozzle type machines in that the paddle dip variable iseliminated. In etching machines using paddles, results are partiallygoverned by the paddle dip, and hence thebath depth becomes criticalsince it regulates the amount of solution carried by the paddles. Inthepresent invention, the

work piece is completely submerged in the bath and the bath depth is notcritical. in the least.

There is also a complete elimination of paddle'speed errors andan'elimin'ation of the necessity of synchronizing the speed of paddleshafts where two or more are utilized, and there is a completeelimination of paddle shaft leaks that might cause a change in the bathlevel and hence in the paddle di'p.

, In the splash type machine, gravity and centrifugal force representdifiicult to control variables. The present invention provides a machinewhich eliminates these factors and provides side wall protection largelyregulated by the gentle flow of bubbles toward the high side of'arevolving work plate, thus, providing fine grained sidewalls having lessshoulder porosity.

Another important object is to provide an etching machine that is simpleand durable in construction, economical to manufacture, eificient inoperation, cost less to maintain, reduces bath cost because of thelonger bath life obtained, and which can be easily operated by an etcherwith only a minimum "of instruction.

The foregoing and numerous other objects and advantages of the inventionwill more clearly appear from the following detailed description of apreferred embodiment, particularly when considered in connection withthe accompanying drawings, in which: a FIG. 1 is a fragmentary,perspective view of the etching machine with the lid raised;

FIG. 2 is a perspective 'view of the etching tank and its appurtenantparts with the lid closed;

FIG, 3 is a top plan view of the machine illustrated in FIG.- 2, butwith the motor removed;

Secondly, the gentle action FIG. 4 is a cross-sectional view of themachine as seen along staggered line 44 of FIG. 3;

FIG. 5 is a cross-sectional view of the tank as seen along line 5-5 ofFIG. 4, and

FIG. 6 is an enlarged, fragmentary front elevational view of theadjustment means.

Referring now by characters of reference to the drawings, and first toFIG. 1, it is seen that the etching tank generally indicated at 11) ismounted in and supported by a body or shell referred to at 11. The body11 serves to enclose certain operating mechanism below the tank in amanner subsequently described, and serves to mount the control panel andinstruments 12 at the front of the machine for ready access by theetcher during operation.

The etching tank 10 is substantially square in configuration havingopposed side walls 13 and right angularly interconnected front wall 14and rear wall 15. The tank 10 includes a fiat horizontal bottom wall 16.The tank It) is adapted to hold a supply of etchant or bath 17 (FIG. 4),the composition and nature of which will be later described.

A lid 20 is connected to the top of rear wall by a hinge 21, the lidbeing adapted selectively to close or cover the open top of tank 10.Attached to opposite sides of lid 20 are a pair of handles 22 tofacilitate raising and lowering of such lid. Because the lid 20 and theparts carried thereby are relatively heavy, a pair of counterbalanceweights 23 are attached to the lid 20 and extend beyond the other sideof the hinge axis 21. Ohviously, the counterbalance weights 23 enablethe lid to be lifted and lowered easily with only a minimum of effort.

Mounted on the top of lid 20 by a hinge 24 is a platform 25. Adjustmentmeans is provided for selectively raising and lowering the platform 25about the axis formed by hinge 24, and hence selectively determine theangular relation of platform 25 relative to lid 20. This adjustmentmeans includes an angle bracket 26 fixed by screws 2'7 to the top of lid20. Formed in the upstanding bracket portion 28 is an elongate verticalslot 29. Extending through slot 29 is a threaded bolt 30 that is fixedto the forward edge of platform 25. A wing nut 31 is threadedly attachedto the end of bolt 30 and engages the vertical bracket portion 28.

In order to adjust the angular position of platform 25, the wing nut 31is loosened and the platform is selectively raised or lowered within therange determined by the movement of bolt 30 in slot 29. To facilitateaccurate and quick placement of platform 25, an angle scale 32 isprovided on the bracket portion 28 alongside the slot 29. The platform25 extends laterally beyond the sides of bracket portion 28, therebyenabling a direct reading on scale 32 of its angular position.

After the platform has been adjusted to the desired angular position,the wing nut 31 is tightened on bolt 30 against the bracket portion 28,thereby clamping the platform 25 and maintaining it in such position.

A drive mechanism generally indicated at 33 and including an electricmotor, speed reducer and interconnecting belt assembly is attached toandsupported on the top of platform 25. The drive means 33 includes adriven shaft 34 that extends downwardly through platform 25 and throughlid 20 into the interior of tank 10 when the lid 20 is closed. It isseen that because the shaft 34 is rotatively mounted on the platform 25,the angular position of such shaft 34 is changed upon adjustment of theangular position of platform 25. The openings in lid 20 through whichthe shaft 34 extends, are of sulficient size to permit such angularadjustment.

Attached to the lower end of driven shaft 34 is a work plate holder 35that is preferably circular in configuration as is best seen in FIG. 1.A zinc work plate 36, shown in FIG. 4 in full lines and illustrated bybroken lines in FIG. 1, is clamped to the underside of holder 35 by apair of adjustable clamps 37 mounted on holder '35. Obviously, theclamps 37 are adjustable toward and away from each other in order toaccommodate work plates 35 of different sizes. In the preferredembodiment, and for reasons later discussed, the diameter of holder 35is at least 4 to 8 inches larger than the greatest dimension of the workplate 36 held by clamps 37.

When the lid 20 is closed over tank 10, the holder 35 maintains the Workplate 36 in an inclined position completely submerged in the etchingbath 17. The work plate 36 is held in substantially parallel relationwith the platform 25 and is angularly adjusted relative to the bathlevel 40 upon angular adjustment of platform 25. The preferred angularposition of work plate 36 during etching operation in order to obtainoptimum results is from two (2) degrees to twelve (12) degrees withrespect to the substantially horizontal bath level 40.

While submerged completely under the etching bath 17 in an angularrelation to the bath level 40, the drive means 33 operates throughdriven shaft 34 to rotate the work plate 36. It has been found thatoptimum etching results are obtained when the work plate 36 is rotatedfrom one-third /3) revolution per minute to ten (10) revolutions perminute.

A ventilating tube 41 is carried by an opening through the forward wall14 above the top of bath level 40, the ventilating tube 41 being adaptedto carry off the fumes from the etching bath 17. A door 42 is providedin the ventilating tube 41 for an opening 43 through which etchant canbe introduced into tank 10 to replenish the bath 17 and raise the level40, if desired or necessary.

Located at the bottom of tank 10 and supported on the bottom wall 16 isan aeration assembly including a header 44 that operatively connects andcommunicates with a plurality of elongate, parallel air tubes 45extending transversely of tank 10. Each of the air tubes 45 is providedwith a plurality of small air holes 46 on its underside. A feed line 47is connected to the header 44 and also connects to a centrifugal pump(not shown). The pump, motor and refrigeration unit utilized by thisetching machine are located under the tank 10 and within the body 11.

Air is forced through the air tubes 45 and is emitted through air holes46 in the form of fine bubbles that rise upwardly through the etchingbath 17 and impinge and roll upwardly on the work plate 36. In additionto this etching action of the bubbles as explained later, the airemitted through air holes 46 keeps the bath agitated and thoroughlymixed.

Uniform air distribution is most important in protecting side walls ofthe image evenly on the work plate 36. Accordingly, the air holes 46 areformed in air tubes 45 at particular locations in order that the bubblesrise through the bath 17 and impinge on the work plate 36 in a definitepattern. More particularly, it has been found that for best results andfor minimum of distortion in etching the image, the air holes must bestaggered in a manner such that a circular pattern relative to therotative axis of the work plate 36 is not formed. In other words, theair holes 46 are formed in tubes 45 so that no two air holes are formedthe same distance from the rotative axis of the work plate 36. In FIG.5, it is seen that the arrangement of air holes 46 form a definitespiral pattern.

As is usual in etching machines and methods of etching, the temperatureof the bath 17 must be maintained within certain specified limits inorder to obtain best results. Formed as a part of the bottom tank wall16 are refrigeration coils 50 as is best seen in FIG. 4. Suitableinsulation 51 is provided about such coils 50. Also provided immediatelybelow the bottom tank wall 16 are a plurality of heating elements 52.Upon selective operation of the refrigeration coils 5d and the heatingelements 52, either manually or automatically by control means, the bathtemperature can be maintained at a desired operating point.

It is thought that the operation and functional advantages of themachine and method has become fully apparent from the foregoing detaileddescription of parts, but for completeness of disclosure such operationwill be briefly described; Y

The protective bath additive is merely combined with water and nitricacid to provide aetching bath 17. The chemicals used in the bathadditive are a thick organic oil, a petroleum" solvent and a wettingagent. The heavy oil most widely used is Turkey red oil (sulfonatedcastor oil), while the thin oils or solvent is kerosene or Solvesso 150consisting of by percent volume 1.3% C aromatics, 44.4% C aromatics,25.3% C aromatics, 2.1% C aromatics, 17.7% indans and 6.5% naphthalenes,a product made by Standard Oil Company. Solvesso 150 is a high puritymixture of substituted benzene compounds having the molecular weightsindicated. For example, the designation of 44.4% of C aromatics is therepresentative percentage of substituted benzene compounds of all typesand all isomers which are present having a C molecular Weight. Thiswould of course, involve the benzene ring which has six carbon atoms andadditional side chains to total the additional four carbon atoms withthe necessary hydrogen present for this type,

akyl-aryl sodium sulphonate and 60% sodium sulphate.

The oils plus the wetting agent constitute that which is termed in theart liquid powder or bath additive.

Bath formulation is based on the size of the desired bath. The bath 17that has been used successfully in the present machine and in thepresent method of an etching is a twelve gallon bath consisting of 83%Water, 13% 42 degree Beaum technical grade nitric acid and 4% bathadditive.

In making the particular powderless etching bath, the tank 10 is filledwith water to a preset water gauge and then the nitric acid is added.The Water and acid go into solution. Because the acid heats the Water 10degrees to 20 degrees, the mixture is cooled to about 75 degreesFahrenheit, and then the bath additive is introduced. The air pump isturned on and the air emitted through air holes 46 rise upwardly throughthe bath 17 to provide thorough agitation that mixes the oils, water andacid. Because the mixture is not a true emulsion, the oils will rise tothe surface of bath 17 if theair is turned off. Therefore, it isextremely important that the bath must be agitated before the work plateis placed into the bath or else the oil will cover the plate and attachitself so completely that the acid cannot etch the plate at all.

Assuming that the print on the work plate is properly exposed, developedand burnt-in, the first step is to paint out the large bare zinc areaswith any lacquer preparation that will stand up under the acid and theoils in' the powderless etching bath 17. This space painting is notabsolutely essential to etching and will have little effect upon theactual etching process. Then, the dry plate is brushed thoroughly with aprepared mixture of nitric acid, water and a metal cleaning solution inorder to remove scum. This de-scum solution is flushed otf of the workplate and then the print is brushed or sponged with a pre-etch solutionconsisting of nitric acid and water. After pre-etching the work plate,the plate is flushed with water prior to gumming. Then, a few drops of aregular gum arabic solution is dropped on the wet plate and brushed overthe entire surface, thus eliminating oxidation and keeping the platefrom drying out while being attached to the plate holder 35.

The air pump is started in order to cause agitation of the bath 17before the lid 20 is closed. It is assumed 5 that the platform 25 hasbeen adjusted as described previously in order to maintain the workplate 36 at the desired angle in the etching bath 17 when the lid 20 isclosed.

The lid 20 is then closed, and the work plate 36 is com rotate the workplate. 36 at the desired revolutions per.

minute.

The zinc work plate has an afiinity for the oil constituting the bathadditive and such oil forms a film over the surface. This film is porousand it is removed by the action of the acid through the pores to a minordegree. However, the major etching action is accomplished by the airbubbles that move upwardly. through the bath 17 into impingement withthe work plate 36. As the air bubbles engage the work plate 36 and roll.upwardly thereon from the low side to the high side because of the plateinclination, the air bubbles remove the oil in the open areas to beetched, allowing the acid to act. As any bubble moves along the .workplate, the bath following the bubble again covers the plate and theprotective oil or bath additive again provides side wall protection.

Furthermore, as a result of the bubbles moving upwardly on the workplate and because of the rotation of such Work plate, the bubbles flowfrom the low side to the high side of the plate in a curved path acrossthe plate. The curve of the flow is counter to the direction of theplate rotation because the flow is in one direction and the plate isturning away from the flow. The curve is determined by the speed ofrotation and the size of the work plate. With the work plate tiltedthere is less action on the side opposite to the direction of the flow.The great er side wall protection is formed on the high side of theimage, and because rotation of the .Work plate puts all sides of allimages on the high side, the side Walls are evenly protected. Thisproduces better protection and again aids in producing shoulders or sidewalls with less porosity. When porosity is eliminated or greatlyreduced, halftone etching, especially fine line halftones, take on addedquality.

It has been found that the plate holder 35 must have a four to eightinch larger diameter than the diagonal of the largest Work plate 36 tobe etched. Without the larger holder, air bubbles striking the workplate at the plates low position tend to be forced off of the plate in adirection opposite to the plate flow and it is also found that thebubbles leaving the work plate on the high side tend to pick up speedand distort the protection. With the larger holder 35 as defined above,bubbles on the low side are fed to the work plate 36 evenly and theaccelerated speed at the top is on the holder 35 rather than on the workplate.

It is seen that the air trapped under the revolving work plate 36 istrying to escape at all times. Even though the bubbles flow to the topof the plate, there can be more air trapped than can flow off at thehigh side. If the bubbles merge, and they certainly do if the plate isperfectly flat rather than being inclined, air'pockets tend to insulatethe plate from the bubble action. If this happens, the protectivechemicals in the bath attach themselves to the plate, thereby formingcone {shaped bumps on the plate bottom. The ideal plate angle then ispartially governed by the air volume, i.e., the bubbles must impinge androll up the work plate 36.

Although the invention has been described by making detailed referenceto a single preferred embodiment, such detail is to be understood in aninstructive, rather than in any restrictive sense, many variants beingpos sible Within the scope of the claims hereunto appended.

We claim as our invention:

1. The method of etching the exposed metal areas of a metallic printingplate coated with a protective resist comprising the steps of subjectingthe plate to submersion face down in a powderless etching bathcontaining a protective bath additive, inclining the plate relative tothe bath level, rotating the plate, and passing air in the form ofbubbles upwardly'through the bath so as to impinge and roll upwardly onthe face of the inclined and submerged plate.

2. The method of etching the exposed metal areas of a zinc printingplate coated with a protective resist comprising the steps of subjectingthe plate to submersion in a powderless etching bath containing anaqueous solution of nitric acid and an additive which includessulfonated castor oil, a kerosene solvent and a wetting agent, incliningthe plate relative to the bath level, rotating the plate, and passingair in the form of bubbles upwardly through the bath so as to impingeand roll upwardly on the face of the inclined and submerged plate.

3. The method of claim 2 in which the bubbles are formed so that theyrise in laterally staggered spaced relation to the rotative axis of theplate. A

4-. The method of claim 2 in which the bubbles are passed upwardlythrough the powderless etching bath before the plate is submerged.

5. The method of claim 4 in which the bubbles rise in a spiral patternfrom the rotative axis of the plate.

References Cited in the file of this patent UNITED STATES PATENTS764,082 Thorpe July 5, 1904 1,016,487 Freer Feb. 6, 1912 1,081,289Albert Dec. 16, 1913 1,267,754 Flick May 28, 1918 1,313,233 Grass Aug.12, 1919 1,340,975 Page May 25, 1920 1,776,535 Bekk Sept. 23, 19302,640,765 Easley et al June 2, 1953 2,776,512 Guenst Jan. 8, 19572,846,295 Patterson et a1. Aug. 5, 1958 FOREIGN PATENTS 372,593 FranceFeb. 22, 1907 936,984 1948 France Feb. 23,

1. THE METHOD OF ETCHING THE EXPOSED METAL AREAS OF A METALLIC PRINTINGPATE COATED WITH A PROTECTIVE RESIST COMPRISING THE STEPS OF SUBJECTINGTHE PLATE TO SUBMERSION FACE DOWN IN A POWDERLESS ETCHING BATHCONTAINING A PROTECTIVE BATH ADDITIVE, INCLINING THE PLATE RELATIVE TOTHE BATH LEVEL, ROTATING THE PLATE, AND PASSING AIR IN THE FORM OFBUBBLES UPWARDLY THROUGH THE BATH SO AS TO IMJPINGE AND ROLL UPWARDLYTHROUGH THE BATH SO AS TO IMPINGE AND ROLL UPWARDLY ON THE SURFACE OFTHE INCLINED AND SUBMERGED PLATE.